rt2x00: convert rt2800_register_read return type

This is a semi-automated conversion to change rt2800_register_read
to return the register contents instead of passing them by value,
resulting in much better object code. The majority of the patch
was done using:

sed -i 's:\(rt2800_register_read(.*, .*\), &\(.*\));:\2 = \1);:' \
       's:\(rt2800_register_read_lock(.*, .*\), &\(.*\));:\2 = \1);:' \
	drivers/net/wireless/ralink/rt2x00/rt2800lib.c

The function itself was modified manually along with the one remaining
multi-line caller that was not covered automatically and the indirect
reference.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>